1. Field of the Invention
The present invention relates to a television receiver, and more perticularly to a time base inversion method of chroma signals and a circuit therefor.
2. Description of Prior Art
Recently a technique to obtain mirror images on a display screen of, e.g., a televison receiver has been developed. The mirror images are generally obtained by inverting the time base of video signals in a horizontal period.
In the NTSC system, for example, a one horizontal period signal of television signals is formatted as shown in FIG. 1 which shows a relation of a composite video signal VA (see FIG. 1(A)) to its modulation time base in the one horizontal period. A video signal V1 to be displayed on each one horizontal line is assigned a video signal period T2 of the one horizontal period signal, that is, in a predetermined period defined by codes b and c on the modulation time base. Now, codes a, d, and codes b and c, previously mentioned, shall be applied on the modulation time base, for defining leading ends or trailing ends of the one horizontal period or respective periods in the one horizontal period. Video signal V1 in video signal period T2 constitutes composite video signal VA, together with a horizontal synchronizing signal SYNC and a burst signal BU both assigned in leading blanking period T1.
Generally the mirror images are obtained by converting composite video signal VA so that the time base of video signal V1 assigned in video signal period T2 is inverted. Then another composite video signal VB as shown in FIG. 1(B) is obtained in reference to its modulation time base. Composite video signal VB as converted is constituted by burst signal BU, horizontal synchronizing signal SYNC, as they are in FIG. 1(A) and a video signal V2 whose modulation time base is inverted in contrast to video signal V1 in FIG. 1(A). That is, horizontal synchronizing signal SYNC and burst signal BU in leading blanking period T1 are not changed, but video signal V1 in video signal period T2 is changed to video signal V2. For example, signal states of video signal V2 at its leading end c and trailing end b correspond to the signal states of video signal V1 at its trailing end c and leading end b, respectively, where, leading end b and trailing end c of video signal V1 are reversed respectively to leading end c and trailing end b of video signal V2. Therefore, the modulation time base of video signal V1 is inverted with respect to the time sequence in video signal period T2.
The conversion of video signal V1 (FIG. 1(A)) to video signal V2 (FIG. 1(B)) for obtaining the mirror images can be carried out by using memory devices to store composite video signal VA of the one horizontal period and address control means for the memory devices.
A writing of the composite video signal data into the memory devices is carried out in order from the entire extent of the one horizontal signal period in FIG. 1(A), i.e., from its leading end a through its trailing end d. A reading of the stored composite video signal data out of the memory devices is carried out in the normal order for the data in leading blanking period T1, i.e., from its leading end a through its trailing end b, in a reversed order from the data in video signal period T2, i.e., from its trailing end c through its leading end b, and in the normal order for the data in trailing blanking period T3, i.e., from its leading end c through its trailing end d. Therefore, when the read sequence in video signal period T2 is carried out in the inverse direction to the write sequence, an inversion of the time base of the video signal (hereinafter referred to as the inversion in leftside right) is carried out. As mentioned above, video signal V2 (FIG. 1(B)) as inverted in leftside right can be obtained.
However, there are serious problems in the conventional systems for obtaining the mirror images in regard to a chroma signal in the video signal. The problems occur due to a phase disturbances of the chroma signal.
Referring now to FIG. 2, the problems will be explained below in detail. FIG. 2(A) shows an example of the chroma signal before the video signal is inverted in leftside right, in which a modulation angle for the chroma signal repeats the phase transition order of color difference signals R-Y and B-Y, i.e., the phase order of (R-Y), (B-Y), -(R-Y) and -(B-Y). When a read phase agrees with the phase axis of the R-Y signal, the phase of the chroma signal is converted in another phase transition order corresponding to the phase order of (R-Y), -(B-Y), -(R-Y) and (B-Y) as shown in FIG. 2(B) upon the inversion of the video signal in leftside right. As a result, the signs of (B-Y) components of the chroma signal can be inverted so that the phase transition order of the chroma signal V2 as inverted becomes different from the original order. Further, in case the read phase agrees with the phase axis of the B-Y signal, the phase of the chroma signal is converted in the order of -(R-Y), (B-Y), (R-Y) and -(B-Y) as shown in FIG. 2(C), so that the signs of the (R-Y) components of the chroma signal can be inverted, and the phase transition order of the chroma signal V2 as inverted also becomes different from the original order.
As a result, in displaying the mirror images on a display screen, the mirror images are characterized by a serious color turbulence.